Luminescent materials are increasingly being used to authenticate a variety of objects. Luminescent materials convert energy from excitation light of a predetermined wavelength into emission light of one or more different wavelengths. Most luminescent materials can be excited with excitation light that comprises different wavelengths. Some luminescent materials emit emission light having two or more different wavelengths even if they are excited with excitation light of only one wavelength.
If the emitted radiation has a longer wavelength than the excitation radiation, it is called “Stokes” or “down-converting” luminescence. On the other hand, if the emitted light has a shorter wavelength than the excitation light, it is called “anti-Stokes” or “up-converting” luminescence.
There are two different types of luminescence: fluorescence and phosphorescence. Fluorescence is the instantaneous emission of emission light upon irradiating a fluorescent material with excitation light. In phosphorescent materials, on the other hand, the emission is delayed relative to the excitation, so that phosphorescent materials phosphoresce or glow after the excitation light has been turned off. The phosphorescence duration and the decay of the phosphorescence are material-specific.
A method for authenticating a marking, i.e. for determining whether a marking comprises a specific photoluminescent material, is known from EP 1 158 459 A1, which discloses the following steps:                exciting the marking with at least one excitation light pulse,        after the excitation, measuring intensity values of the emission light at predetermined time intervals,        forming a function representing the intensity as a function of time, and        comparing the function with at least one reference function, wherein the function and the reference function are normalized prior to the comparison.        